The present invention relates to check valves for selectively interrupting fluid flow through the aperture of a valve seat. More particularly, the present invention relates to an improved pilot check valve that selectively moves a valve plunger to close the valve seat aperture.
Check valves control the flow of fluids through a conduit or housing. A valve plunger contacts a valve seat to block fluid flow through the aperture and is selectively withdrawn to permit fluid flow through the aperture. In a simple check valve, a spring urges the valve plunger against the valve seat, and the force exerted by the fluid against the valve plunger compresses the spring to unseat the valve plunger from the valve seat. When the fluid pressure subsides, the spring reurges the valve plunger against the valve seat.
A pilot check valve differs from a simple check valve and includes a mechanism for positively closing the valve plunger against the valve seat. One commercial embodiment of a hydraulic pilot check valve is illustrated in FIG. 1, wherein valve plunger A is held against valve seat B by the force exerted by spring C. Valve piston D is positioned between spring C and valve body E and is in contact with fluid in cavity F. Spring C is positioned in cavity G which communicates with the hydraulic fluid at the outlet, and seal H is positioned between cavities F and G in the sealing gap between valve piston D and valve housing I. When hydraulic fluid contacts valve plunger A, spring C is compressed to withdraw plunger A from contact with valve seat B so that the hydraulic fluid can flow from the inlet to the outlet. This position describes the normal open valve configuration which permits hydraulic fluid flow from the inlet to the outlet.
When the fluid in cavity F is pressurized from the common pressure source, valve piston D should theoretically move to compress spring C, which in turn should move to urge valve plunger A into contact with valve seat B. However, this desired movement is hindered because the fluid pressure in cavities F and G is the same. Other than the force exerted by spring C, there is no differential force acting to urge valve plunger A toward valve seat B. This problem limits the successful operation of the hydraulic check valve design shown in FIG. 1. Consequently, the valve plunger may not close the valve seat aperture to prevent fluid flow through the check valve.
Accordingly, there is a need for an improved pilot check valve that can selectively close a valve seat regardless of the valve inlet fluid pressure. The pilot check valve should reliably close the valve seat in response to a selected control operation.